An ATM network transports data signals requiring various bandwidths using standard size cells, each cell being formed of a predetermined number of bytes. Each cell has a header which identifies what circuit it is to be routed to, and therefore identifies the destination of the cell. Cells are typically related to each other and form packets, wherein all cells of a packet have the same circuit identifier, referred to as a VPI/VCI value.
Variable length data packets are segmented into cells at the point where they enter the ATM realm, and the cells are reassembled into packets at the point where they leave the realm. Circuitry which performs both of these functions are referred to as SARs (segmentation and reassembly).
A SAR typically contains a buffer memory, wherein packets are stored as they are segmented or reassembled. The buffer memory is accessed by a microprocessor via a bus. However, certain kinds of data cannot tolerate storage in a buffer memory, such as a data stream carrying video data from a video telephone and a data stream carrying audio from the same video telephone. In the event either the audio or video data is delayed due to reassembly of packets in the buffer memory, lip sync can be lost. In the event either the audio or video data is delayed in order to synchronize with the other, delays in the transmission of data can disrupt a conversation between two parties, since the timing of remarks and responses can make the conversation erratic and pauses introduced by the system can convey wrong information.
Information about ATM networks and systems may be found in the following publications: PM5346 S/UNI-Lite Data Sheet, PMC-Sierra, Inc. Issue 3, May, 1994; SATURN Compatible Interface for ATM PHY and ATM Layer Devices, PMC-Sierra, Inc., Issue 3, Nov. 1994; ATM User-Network Interface Specification, Version 3.0", ATM Forum, 1993; W. Kelt, G. Fedorkow, C. Bailey, P. Regache, I. Chaudhri, B. Loyer, D. Young, V. Little, S. Christensen, G. Garg and R. Curtis, "An ATM PHY Data Path Interface", ATM Forum, Contribution AF 93-0940, (UTOPIA Specification), 1993.